Hairy-cell leukemia: Leukemia marked by an abundance of large, mononuclear abnormal cells with numerous irregular cytoplasmic projections that give them a hairy appearance in the bone marrow, spleen, liver, and peripheral blood. Half-life: Length of time required for the decay of one half of a radioactive material. During decay different radioactive material emits various kinds of particles such as alpha, beta, and gamma before becoming neutral that can vary from fractions of a second to billions of years. Hand-and-foot syndrome: Recurrent painful swelling of the hands and feet occurring in infants and young children with sickle cell anemia. Helicase: An enzyme that unwinds double-stranded DNA or RNA to two single strands. Helper T cells: A type of white blood cell produced by the thymus gland whose presence is necessary for normal levels of antibodies to be produced by B lymphocytes. Helper virus: Replication-competent virus that may be produced during the procedure of gene transfer technique in the packaging cells. Hemangioma: An extremely common benign tumor of blood vessels. Hematocrit: A test used to determine the volume of packed red cells in a blood specimen. Low hematocrit value indicates fewer red blood cells, that is anemia. Hematological: Relating to hematology that is that branch of medical science which studies the morphology of the blood and blood forming tissue. Hematopoietic growth factor: Protein factor that stimulates the production and growth of certain types of blood cells. Hematopoietic: Pertaining to the formation of blood cells. Hematuria: Blood in the urine. Hemoccult II: Trade name for a modification of the guaic test for occult blood in which guaic-impregnated filter paper is used; the test is positive if the specimen turns blue. Hemoglobin: The oxygen-carrying pigment of erythrocytes (the red blood cells). It is a conjugated protein containing four heme groups and globin. Hemopoietic Progenitor Cells: Parent blood-forming cells. Hemopoietic Stem Cells: Blood cell progenitor having the capacity for both replication and differentiation, and giving rise to various morphologically recognizable precursors of different blood cell lines. Hepatocarcinogen: That causes cancer of the liver. Heterochromatin: The state of chromatin, which is tightly coiled, and dark-staining, forming irregular clumps in interphase. It is genetically inactive. Heterocyclic compounds: Chemical agents with ring structures, which contain one or more atoms other than carbon. For example, the bases in DNA and RNA shown below are all heterocyclic compounds. . Heterooligomer: A macromolecule containing two or more subunits when the subunits are different. Heterozygous: Presence of different alleles in a chromosome for a given character. Histocompatibility Antigen: Specific proteins present on the cell surface taking part in transplantation immunity. If a foreign histocompatibility antigen is introduced as a graft from a genetically distinct individual, it will be rejected by a cell-mediated immune reaction. Histological: Pertaining to histology, the study of microscopic structure and arrangement of tissues in organ. Histology: The study of microscopic structure and arrangement of tissues in organ. HLA: Human Leukocyte Antigen, histocompatibility antigens governed by the genes of the HLA complex (the human major histocompatibility complex). Hodgkin's Lymphoma: A form of malignant lymphoma characterized by painless, progressive enlargement of the lymph nodes, spleen, and general lymphoid tissues. Homologous chromosome: Chromosomes, which have genes controlling the same characteristics, one is obtained from the male and the other from the female parent. Hormone replacement therapy: The combination therapy of estrogen and progestogen is termed as hormone replacement therapy or HRT. Hormones: Chemical compounds produced in specific glands (the endocrine glands) and is carried through blood to a distal part of the body for action. H-ras: A point-mutated proto-oncogene that is found in melanoma and in carcinoma of colon, lung and pancreatic tissue. Human Chorionic Gonadotropin (hCG): Carbohydrate-containing peptide hormone produced by the fetal placenta. It can be detected in the maternal urine within days after fertilization and thus provides the basis of the most commonly used pregnancy test. Humoral (immunity): Immunity produced by antibodies present in blood as opposed to cellular immunity. Humoral Immunity: Immune reaction produced by the circulating antibody synthesized by the B-lymphocytes. Huntington's disease: Relatively common autosomal dominant disease characterized by chronic progressive chorea (the ceaseless occurrence of a wide variety of rapid, highly complex, well coordinated jerky movements that are involuntary) and mental deterioration terminating in dementia; the age of onset is variable but usually occurs in the fourth decade of life. Hybridization: The act of producing hybrid. In molecular biology two complementary strands of DNA when annealed form a hybrid, or when an RNA strand anneals with a complementary DNA strand. Hybridoma: A somatic cell hybrid formed by the fusion of normal lymphocytes and tumor cells; the resulting cell will produce the same secretion as the normal parent cells and proliferate indefinitely in culture like the parent tumor cells. Hydrocortisone: The major glucocorticoid produced by adrenal cortex has effect on blood pressure. This corticoid is used for the treatment of inflammation, allergies, rheumatoid arthritis etc. Hydrolase: Enzyme that catalyzes the hydrolytic cleavage of a chemical bond with the addition of water, for example gycosidase, phosphatase, nucleotidase, peptidase. Hydrolytic enzymes: Also known as hydrolases, a class of enzymes (EC Class 3) catalysing hydrolysis of a variety of bonds, such as esters, glycosides, peptides. Hydrolysis is the process where the splitting of a compound into fragments is achieved by the addition of water, the hydroxyl group being incorporated in one fragment and the hydrogen atom in the other. Hydrophobic: Not soluble in water, lacks polar groups. Hypercalemia: A medical condition in which abnormally high concentration of calcium compounds are found in the bloodstream. Hypercholesterolemia: Excess of cholesterol in the blood. Hypereosinophilia: An abnormally high number of eosinophils in the blood. Normally, eosinophils constitute 1 to 3% of the peripheral blood leukocytes, at a count of 350 to 650 per cubic millimeter. Eosinophilia can be categorized as mild (less than 1500 eosinophils per cubic millimeter), moderate (1500 to 5000 per cubic millimeter), or severe (more than 5000 per cubic millimeter). Hypernephroma: The most common form of kidney cancer, which occurs when the cells lining the renal tubule undergo cancerous changes. Hyperplasia: The abnormal multiplication or increase in the number of normal cells in a tissue. Hyperplastic disorder: Disorder that is characterized by abnormal multiplication or increase in the number of normal cells in a normal tissue. Hyponatremia: Deficiency of sodium in the blood. Hypoxia: Reduction of oxygen supply to tissue below physiological levels despite adequate perfusion of the tissue by blood.